1College of Computer Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu 210016, China2School of Computer Science & Technology/School of Software, Nanjing University of Posts and Telecommunications, Nanjing, Jiangsu 210023, China3School of Information and Statistics, Guangxi University of Finance and Economics, Nanning 530003, China

Received 21 November 2013; Accepted 23 February 2014; Published 15 May 2014